In order to sign PDF documents in a more legally admissible way, please note the following points:

Secure transfer of the signature (i.e. its biometric data) to the PDF document and its secure storage therein (the signature may at no time be copied or otherwise used).

The identification mark (the signature) must be inextricably linked with the document content upon signing the PDF file: i.e. later modifications to the document must render the signature invalid.

Self-protection for the operator (in a legal sense) must be ensured. This means that operators must not have the opportunity to decrypt the biometric data of the signature or extract it from the signed PDF file in such a way that enables them to use it to sign another document. Otherwise, they would not be able to prove that the signer actually intended to sign this document due to burden of proof (analogous to paper documents) on operators.

The identification mark (i.e. the signature) must be verifiable independently of technology and manufacturer. This is necessary a) to provide demonstrable and credible evidence in court and b) to assure users that the electronically signed documents can also be used in the future.